I'm a chemist by training and not a bioligist and freely admit to knowing nothing about elasmobranchs. However, my point is not about chemicals like progesterone that have a purpose, but rather the chemicals that don't have a purpose. Evolution would predict that the body should be churning them out all the time but they're just not there.I've always found that chemists have a lot more doubt about the current theory of evolution than biologists. It sounds like a great idea at the big picture level but it runs into all sorts of problems at the molecular level.
You should take endocrinology--you'd love it. Actually many of our biochemicals are manufactured in stages, e.g, cholesterol to progesterone to 5-alpha dihydrotestosterone to testosterone, etc., etc. For some of these intermediary stages function has been found. For others it has not been elucidated. These are some intermediary stages in neurochemistry that are so short-lived that neuroscientists are sure they are not identifying them all. Then there are the neuoropeptides secreted by the hypothalamo-hypophyseal axis. One such has been isolated long ago (adrenalcorticotropic hormone) but still today even the best methods still produce a "contaminated" product that has other (unidentified) neuropeptides present. These contaminants are bioactive.